POLAR
PLASMA WAVE INVESTIGATION: SCIENCE NUGGETS
- COHERENT STRUCTURES OBSERVED BY THE POLAR PLASMA WAVE INSTRUMENT
The waveform interferometry data from the POLAR Plasma Wave Instrument
have been used to estimate the typical parallel scale sizes
(100-1000 m), velocities (1000 km/s) and the bi-directional propagation
of the coherent electric field structures previously observed by the
GEOTAIL and FAST spacecraft in magnetospheric regions with bursty
broadband electrostatic noise spectral signatures. These structures
are found to correspond to regions of positive charge at high altitudes
in the earth's polar magnetosphere. The determination of scale size
and velocity estimates now make it possible to quantitatively test
theories of coherent structures.
Franz, Jason R., Paul M. Kintner and Jolene S. Pickett, POLAR
observations of coherent electric field structures, Geophys. Res. Lett.,
Vol. 25, 1277, April 15, 1998.
- SOLAR WIND DYNAMIC PRESSURE AS A DRIVER FOR CHORUS EMISSIONS
Coherent electromagnetic chorus emissions, the most intense of
all naturally-generated plasma waves in the earth's magnetosphere,
have been observed to be turned on/off in direct association with
sudden fluctuations in solar wind dynamic pressure and southward
turnings of the interplanetary magnetic field (IMF) associated with
the impact of a magnetic cloud on the magnetosphere. High resolution
waveform data from the POLAR Plasma Wave Instrument have shown a
fast and direct response of the chorus to sudden magnetospheric
compression/relaxation and to small, quick variations in the IMF on
short (<60 second) time scales. These findings cast doubt on
previously held mechanisms for chorus generation and requires that
new and faster operating mechanisms may be responsible for the
emission of these intense plasma waves, which drive morningside
pulsating and diffuse aurorae.
Lauben, D.S., U.S. Inan, T.F. Bell, D.L. Kirchner, G.B. Hospodarsky,
and J.S. Pickett, VLF chorus emissions observed by POLAR during the
January 10, 1997 magnetic cloud, Geophys. Res. Lett., Vol. 25, 2995,
August 1, 1998.
- BROADBAND PLASMA WAVES IN THE POLAR CAP BOUNDARY LAYER
Broadband plasma waves have been found to bound the dayside polar cap
magnetic field lines. These emissions occur on nearly every apogee
Polar pass with a 96% occurrence rate and are well-correlated with
enhanced fluxes of 10 to 100 eV protons, doubly charged helium and
singly charged oxygen ions. In the noon sector there is a strong
relationship between the enhanced fluxes of energetic ionospheric
and magnetosheath ions and intense wave energy, suggesting that
these waves may be responsible for ion heating near the cusp region.
The greater power in the low frequency end of the wave spectra
indicates that these waves will preferentially energize oxygen ions
over protons, causing the heating of ionospheric oxygen on these
field lines. If the interplanetary magnetic field turns southward,
interconnection with the earth's magnetic field will occur, leading
to oxygen ion transport into the earth's magnetotail. Subsequent
injection into the nightside magnetosphere could lead to the delayed
enhancement of oxygen ions in the ring current. Ion heating by these
broadband waves provides one possible mechanism to explain plasmasheet
ion composition during a storm main phase.
Tsurutani, B.T., G.S. Lakhina, C.M. Ho, J.K. Arballo, C. Galvan,
A. Boonsiriseth, J.S. Pickett, D.A. Gurnett, W.K. Peterson, and
R.M. Thorne, Broadband plasma waves observed in the polar cap
boundary layer: Polar, J. Geophys. Res., Vol. 103, 17,351, August 1,
1998.
- EQUATORIAL SOURCE LOCATION OF CHORUS EMISSIONS
Whistler-mode chorus is generated within a few degrees of the
geomagnetic equator. A spatial survey of Poynting flux measurements
computed using high-resolution electric and magnetic field waveform
data from the Polar Plasma Wave Instrument provides the first direct
and unambiguous evidence that chorus is generated near the geomagnetic
equator. Chorus is shown without exception to propagate away from the
geomagnetic equator. No chorus emissions were observed to propagate
toward the equator as might be expected from high-latitude
magnetospheric reflections. The absence of a reflected component
indicates that chorus is absorbed before reflection. These results
indicate that previously suggested chorus generation mechanisms
requiring multiple passes through the source region are not valid.
LeDocq, M. J., D. A. Gurnett, and G. B. Hospodarsky, Chorus source
locations from VLF Poynting flux measurements with the Polar
spacecraft, Geophys. Res. Lett., Vol. 25, 4063, November 1, 1998.